An alkene is an unsaturated hydrocarbon that contains at least one carbon–carbon double bond (C=C). The double bond is the functional group and is the site of the characteristic addition reactions of alkenes, making them far more reactive than alkanes. An acyclic (open-chain) alkene has the general formula CnH2n, two hydrogens fewer than the saturated alkane of the same carbon count.

烯烃是至少含有一个碳碳双键(C=C)的不饱和烃。该双键即为官能团,是烯烃特征加成反应的发生部位,使其反应性远高于烷烃。开链烯烃的通式为 CnH2n,比相同碳数的饱和烷烃少两个氢。